Helper-dependent phage P4 is a satellite virus depending on temperate phage P2 as a helper. P2 provides the proteins for satellite phage head and tail synthesis, as well as cell lysis. P4 causes P2 head proteins to form a small head which is not found during normal P2 head morphogenesis. Thus P4 causes specific head size determination. P4 induces an RNA polymerase activity which may initiate DNA replication by synthesizing polyriboguanylic acid. P4 amber mutants and gene alpha do not synthesize DNA nor produce this enzyme. We have developed a P4 in vitro replication system in order to purify the alpha gene product and to detemine its relation to DNA synthesis and to the RNA polymerase activity.